1. Field of the Invention
This invention relates generally to the control of a machine by a digital signal generated by a computer and converted to an analog signal, and more particularly relates to a glitch detector and trap.
2. Description of the Prior Art
Digital computers are frequently used to activate, operate or drive various analog systems. Hence, an analog signal may be derived from the digital signal which, in turn, is used to operate a machine. Among such analog systems are computer controlled machine tools or flight simulation systems where a base platform is moved to simulate a flight.
In such analog systems, a digital computer is interfaced to a mechanical servo-control device or machine by means of a digital-to-analog (D/A) converter. The output of the converter is generally a time variant function which commands the servo system. However, the digital-to-analog converted output is not a smooth and continuous function. Rather it consists of discrete steps corresponding to the sample periods of the converter. The steps are generally small in magnitude and short in duration such, for example, as 20 milliseconds. Obviously, the duration of each step is small compared to the frequencies or passband of the machine or function to be controlled.
In such systems, a serious problem may arise. This may, for example, be due to the fact that some sample or step of the analog signal is erroneous because a significant data bit is either lost or miscalculated. This will result in a large amplitude spurious spike of the analog signal which is usually called a glitch and appears at the output of the converter. It will be realized that such spikes are highly undesirable and may cause unwanted, large physical motions which may be highly disagreeable to a trainee on a flight simulation system.
In this connection, reference is made to a patent to Salmet U.S. Pat. No. 3,375,445. The patent discloses a system to be used in a pulse position modulation system. According to the system, the through-put is normally gated off and operates only when a single pulse is received. However, the system is prevented from operating when multiple pulses are received. To this end, two channels are connected to the output of an amplifier. One channel includes a delay line; the second channel operates to cut off a gate if more than one pulse is received in a certain period of time.
Furthermore, the patent to Prentky et al. U.S. Pat. No. 3,665,327 discloses an inhibiting gate to exclude extraneous signals. The noise discriminator described is all digital in nature and does not generate an analog signal.